A hard disk drive or hard drive (HDD) is a storage drive used for storing and retrieving digital information. Hard drives come in multiple data storage capacities generally resulting in different thicknesses. Hard drives of the same sized (diameter) disks typically increase in thickness as storage capacity increases. This is because disks are generally stacked on top of each other to increase capacity. At the same time, customers demand size efficient enclosures to house their hard drive. Current common disk sizes include 1.8 inches, 2.5 inches, and 3.5 inches, though other disk sizes can be used.